In random access information storage systems, a read/write head must be moved periodically and reliably in track seek operations between spaced apart data tracks on the storage disk to access desired information sectors randomly located on the disk tracks. Various servo control systems are known for controlling the acceleration and deceleration of the head during the track seek operation. Ideally, movement of the head is controlled in such a manner that it arrives over the target track at the end of track seek with zero velocity. At this time, control over movement of the head is handed off from the track seek servo control circuit to the track follower servo control circuit to maintain the track sensor located in the head centered on the desired track during read or write operation.
It is well known to use dedicated servo control signals recorded on a servo disk or on the data disk to provide clean feedback control signals for reliable, continuous feedback servo control of head movement during seek operation. However, with ever increasing disk storage capacities, such as the case with optical and magneto-optical systems, servo control requirements become more stringent and dedicated servo control signals are no longer feasible in most cases. Thus feedback control is derived from tracking error signals generated by the track sensor from transitions between grooves and lands formed on the disk surface. Generally speaking, tracking error signals of this type are of much poorer quality than tracking error signals derived from dedicated servo control signals. In such situations, continuous feedback control is no longer feasible and sampling systems are used that make control of head movement less reliable, particularly when approaching the target track at the end of the track seek operation. One consequence is that the read/write head, on occasion, may arrive at the target track with a non-zero velocity and, in particular, a velocity that is too high for the track follower servo circuit to effectively take over the positioning control of the head to hold the head on the target track. If this occurs, the head will continue to move beyond the target track until stopped, either at the extreme inward or outward travel of the head or by some mechanism that recognizes the out-of-control condition and brings the head to a halt before it reaches its extreme travel boundary. Once stopped, the new track location can be read and an appropriate return seek executed in known manner to move the head back to the target track.
It is therefore an object of the present invention to provide a system and method for detecting overshoot of a read/write head beyond the target track at the end of a track seek operation and for bringing the head motion to a low enough velocity that will allow effective acquisition of track follower servo control of head positioning.